338 THE MAMMALIAN EMBRYO. [CHAP. 



DIAGRAMMATIC LONGITUDINAL SECTION OF THE OVUM TO 



WHICH THE EMBRYO (FiG. 110 A.) BELONGED. (After His.) 



am. amnion ; Nb. umbilical vesicle. 



dicated. The amnion (am) was completely formed, and 

 vitelline vessels had made their appearance. 



Two embryos described by Allen Thomson are but 

 slightly older than the above embryo of His. Both of 

 them probably belong to the first fortnight of preg- 

 nancy. In both cases the embryo was more or less 

 folded off from the yolk-sac, and in one of them the 

 medullary groove was still widely open, except in the 

 region of the neck (Fig. 110 B). The allantoic stalk, if 

 present, was not clearly made out, and the condition of 

 the amnion was also not fully studied. The smaller of 

 the two ova was just 6 mm. in its largest diameter, and 

 was nearly completely covered with simple villi, more 

 developed on one side than on the other. 



In a somewhat later period, about the stage of a 

 chick at the end of the second day, the medullary folds 

 are completely closed, the region of the brain already 

 marked, and the cranial flexure commencing. The 

 mesoblast is divided up into numerous somites, and the 

 mandibular and first two branchial arches are indicated. 



